i have a vague recollection of this, it was in a book, don't remember which one. The theory goes that its rarely an issue getting air into a turbocharged engine as the turbo is forcing it in. The trouble with a turbocharged lump is getting the gasses out of the cylinder after combustion, this is because of the resistance caused by having to spin the turbine of the turbocharger. In this book there was an ideal ratio between the inlet vs exhaust valve size. It would be good if someone could remember what book it was, its something I'm looking to try out in the near future.